Monthly Highlight

Published: Mar 1, 2018

Categories: Base Peak

Ancient glues

Humans have used glues for several hundred thousand years back to the Middle Palaeolithic period, up to 300,000 years ago, and archaeological finds have indicated that they became more common about 80,000 years ago. One of their key uses was to fix axe heads to handles and their discovery on such old artefacts is evidence of some degree of skill and intelligence, given the fact that glues had to be prepared from plant resins or gums or distilled plant products like pitch and tar.

Evidence for the existence of Palaeolithic glues is rare, so samples must be treated with care to preserve their integrity. One of the key procedures for analysing their compositions is GC/MS. It can identify biomarkers such as abietic acid from conifer resins and betulin from birch bark resins but is is not without its difficulties due to the varying complexities of the glues and their different levels of degradation. In addition, GC/MS of glue extracts destroys the sample and this is not an acceptable outcome when sample quantities are scarce. The alternative analytical methods like vibrational spectroscopy, optical microscopy and SEM-EDX are non-destructive but far less precise than GC/MS.

Improvements in the mass spectrometric approach have been achieved by linking GC/MS with headspace solid-phase microextraction (HS-SPME) which has been demonstrated for birch pitch, but the chromatograms are exceedingly complex to decipher. In 2016, scientists tested HS-SPME with 2D GC/MS on a time-of-flight mass spectrometer, the dual GC columns giving even better component separation. Nevertheless, a complex resin-beeswax sample with hundreds of organic components remained difficult to resolve.

Now, that same team has adapted their method further by fitting a high-resolution TOF mass spectrometer to the 2D GC system, to improve component resolution even more. It has been described by Dries Cnuts and colleagues from the University of Liège, Belgium, Chaminade University of Honolulu and Dartmouth College, Hanover, NH and was reported in Archaeometry.

GC with high-resolution MS

In order to test their method, the researchers prepared some glue samples in the lab, rather than wasting precious archaeological resources. Pinus negra (black pine) resin, Picea abies (Norway spruce) resin, acacia gum and birch pitch were produced and then used alone or in mixtures with beeswax to simulate real samples. They were transferred to glass slides and stored in headspace vials.

The volatile components in the resins were samples onto SPME fibres while heating the vials then the extracted components were analysed in the 2D-GC-TOFMS system using electron ionisation. The mass spectra obtained were searched against two commercial databases to identify the various components.

The complexity of the total ion chromatograms varied with the type of glue, birch pitch being the most complex containing hundreds of compounds. The profiles of black pine and Norway spruce were different, indicating that the sources of glues can be differentiated at the species level due to the variations in content of the terpenoid components. The acacia gum had relatively few peaks, suggesting that a method based on volatile profiles might not work for polysaccharide-based glues.

A principal components analysis of the data for the glue mixtures provided some distinction between the glue types. Birch pitch formed a distinct group while the resins were slightly overlapped due to the natural variation of their constituent terpenoids. The resins could be distinguished from the resin-beeswax mixtures.

In a series of blind tests, the confirmation of resin, pitch or gum was highly successful but it was more difficult to identify the particular plant species within them. The comparisons indicated that each resin had its own unique signature of volatile compounds, indicating that an extensive collection of reference samples would be required. The minimal sample size depended upon the type of glue. Beeswax was also problematical due to its low content of volatile compounds, its presence being completely masked by the volatiles from birch pitch and plant gum.

Glues on flint tools

To direct the research closer to an archaeological context, the glues were also adhered to flint tools which were placed in the SPME vials for extraction. The presence of other organic sediment or the stone itself did not interfere with the glue analyses and six glues were correctly identified.

So, the method holds promise for the analysis of Palaeolithic glues and the research team proposed it as a first line of testing. It is non-destructive, requires small sample sizes and has the advantage of gentle heating, usually to 40-50°C, to maintain the integrity of the ancient samples.

In the future, the experimental conditions for plant gums and beeswax will be optimised to provide more definitive results. In addition, application to artificially aged samples as well as archaeological artefacts will help to refine the experimental protocol. This should be supported by a comprehensive reference database of glues made up from different plant materials such as tar, pitch and gum, as well as fossil materials like bitumen.